Makin' Money the Easy Way! (though possibly also the unethical way)

A friend gave me a little "tip" the other day which last night I had the opportunity to put into practice. Her suggestion was, when you are bartending and someone offers to buy you a drink, you say "yes i would love one" and instead of taking said drink, you take said money that you have just charged the person for said drink. I thought to myself...BRILLIANT!

Here in Puerto Rico, we are taking about making an extra dollar or two on a beer, so i don't *entirely* feel as though i'm stealing. I was pondering last night as i hustled Medallas and rum shot to middle aged men that would i feel bad/unethical about doing this "trick" were I working in a place where drinks were more in $7-10 range? I mean, taking a DOLLAR instead of just taking a beer doesn't seem like a big deal. But what if i were taking TEN dollars instead of a ten dollar drink? Would that be wrong? Is what i am doing WRONG??? I just can't decide. I doubled my tips last night by doing this trick, so it makes it even harder to decide. And, if i decide that it is in fact NOT wrong to do this, then I need to start having people buy me more expensive drinks...